When measuring lengths with the aid of instruments or measuring machines that are equipped with physical length scales, temperature deviations from a reference temperature will constitute a source of error. The temperature that prevails during a measuring occasion will normally deviate from the reference temperature. The reference temperature can normally only be included in particularly well-equipped measuring laboratories. Three types of measurement errors occur when measuring in temperature conditions that are not ideal:
1) Errors which are caused by dimensional changes in the measured object due to linear (heat) expansion. PA1 2) Errors due to changes in the geometry of the machine or corresponding arrangement due to thermal linear expansion. PA1 3) Errors caused by changes in the measurement of the length reference, i.e. the graduated measuring scale, due to linear (heat) expansion. PA1 .delta.=the total linear expansion PA1 .alpha.=the coefficient of linear expansion of the scale PA1 dT=the deviation from the reference temperature PA1 L=scale length.
The present invention relates to a method and to an arrangement for determining and correcting errors of this last mentioned kind.
In some instances, the inventive method and arrangement may also be used to determine and correct errors according to (2) above.
It has earlier been proposed to manufacture scales from material whose coefficient of linear expansion is equal to zero, or close to zero. This proposal, however, is encumbered with several drawbacks. For example, such material is very expensive and creates difficult construction problems when the material is to be incorporated in arrangement and machines of the kind intended here, which are often constructed from material whose coefficient of linear expansion deviates from zero and which has a coefficient of linear expansion which greatly deviates from zero.
It has also been proposed that because the change in length of these scales is often repeatable in these contexts, the temperature of the scale can be measured and the linear expansion then calculated on the basis of the coefficient of linear expansion of the scale material, in accordance with the formula: EQU .delta.=.alpha..multidot.dT.multidot.L
where
One serious drawback with this method, however, is that it is necessary to measure temperature extremely accurately and also to measure temperature at several places along the scale, since the temperature is liable to vary along said scale, and also that the coefficient of linear expansion must be known to extreme degrees of accuracy.